Doxastic logic
Doxastic logic is a type of logic concerned with reasoning about beliefs. The term doxastic derives from the ancient Greek δόξα, doxa, which means "belief." Typically, a doxastic logic uses 'ℬx' to mean "It is believed that x is the case," and the set denotes a set of beliefs. In doxastic logic, belief is treated as a modal operator.
There is complete parallelism between a person who believes propositions and a formal system that derives propositions. Using doxastic logic, one can express the epistemic counterpart of Gödel's incompleteness theorem of metalogic, as well as Löb's theorem, and other metalogical results in terms of belief.
Types of reasoners
To demonstrate the properties of sets of beliefs, Raymond Smullyan defines the following types of reasoners:
Accurate reasoner: An accurate reasoner never believes any false proposition. (modal axiom T)
Inaccurate reasoner: An inaccurate reasoner believes at least one false proposition.
Conceited reasoner: A conceited reasoner believes his or her beliefs are never inaccurate.